Typical examples of display panels are liquid crystal display panels. A liquid crystal display panel includes a lower substrate and upper substrate, wherein each substrate has an electric field-generating electrode at one surface thereof and both substrates face each other with a liquid crystal layer interposed between them.
Such liquid crystal display panels may be classified into passive-matrix liquid crystal display panels and active-matrix liquid crystal display panels depending on methods for switching a liquid crystal molecule array. Particularly, active-matrix liquid crystal display panels are obtained by forming multiple pixels on a liquid crystal display panel and mounting a switching element to each pixel. Thus, it is possible to obtain relatively high picture quality and high response rate by controlling on/off operations of each pixel independently and to provide excellent resolution and moving picture realizing quality. As the switching element, a thin-film transistor (TFT) is generally used.
FIG. 1 is an exploded perspective view showing a general active-matrix liquid crystal display panel and FIG. 2 is a sectional view taken along line II-II in FIG. 1. Hereinafter, structural characteristics of a general liquid crystal display panel will be explained with reference to FIGS. 1 and 2.
A general active-matrix liquid crystal display panel includes a lower substrate 10 and upper substrate 30, wherein each substrate has an electric field-generating electrode at one surface thereof and both electrodes face each other, and a liquid crystal display layer 40 interposed between the lower substrate 10 and upper substrate 30.
The lower substrate 10 includes a plurality of parallel gate lines 14 each outputting a scanning voltage and a plurality of parallel data lines 16 each outputting an image voltage arrayed in a crossing pattern so as to define net-shaped pixel zones P on a first transparent substrate 12 such as glass. Additionally, a thin-film transistor T is disposed at each intersecting point of the gate lines 14 and data lines 16. Further, a first electrode 18 connected to a thin film transistor T with a one-to-one correspondence is mounted on each pixel zone P.
The first electrode 18 serves as working electric field-generating electrode for applying electric voltage to the liquid crystal display layer 40. A portion where the first electrode is disposed becomes a display zone of the liquid crystal display panel and other portions become non-display zones. Additionally, the thin-film transistor T is turned on/off through a signal voltage outputted to the gate lines 14 and serves as switching element for applying the signal voltage outputted from the data lines 16 selectively to the first electrode 18.
Additionally, the upper substrate 30 includes a second transparent substrate 32 such as glass, whose back surface has a color filter layer 36 including a plurality of color filters 36a, 36b, 36c arrayed with adjacent to each other so as to shield a ray of light with a certain wavelength and a second electrode 38 serving as another electric field-generating electrode, in turn.
Further, a black matrix 34 is disposed between the second transparent substrate 32 and color filter layer 36, the black matrix 34 serving to prevent a light leakage phenomenon that may be generated at the boundary zones of adjacent color filters 36a, 36b, 36c and to interrupt light incoming into the thin-film transistor T.
The liquid crystal layer 40 is interposed between the lower substrate 10 and upper substrate 30.
In the above-described liquid crystal display panel, the liquid crystal layer 40 should maintain a constant thickness between both substrates. To accomplish this, a spacer is disposed between both substrates.
In general, spacer beads such as glass beads, plastic beads, etc., having a predetermined particle diameter are used in liquid crystal display panels and touch panels in order to maintain a constant distance between an upper substrate and lower substrate. However, because such spacer beads are applied randomly, they may be present at efficient pixel portions through which light is transmitted, resulting in distortion of alignment of liquid crystals and a drop in contrast ratio. Additionally, such spacer beads are problematic in that they can move freely in a liquid crystal cell and can be distributed non-uniformly, resulting in generation of undesirable marks due to the agglomeration of spacer beads in some cases.
To solve these problems, it is suggested to form a spacer by lithography. Such lithographic methods include applying a photosensitive resin composition onto a substrate, irradiating a predetermined portion of the substrate with ultraviolet rays by using a desired mask, carrying out development with an alkaline developer solution to form a spacer pattern having a desired shape, and then carrying out a final curing step to stabilize the pattern. Herein, a spacer formed by the above-described method is referred to as “patterned spacer”.
FIG. 2 is a sectional view taken along line II-II in FIG. 1 and shows a cross section obtained by cutting the liquid crystal display panel as shown in FIG. 1 along the part having a spacer, after the lamination of the lower substrate 10 and upper substrate 30.
As shown in FIG. 2, the second electrode 38, color filter layer 36, black matrix 34 and the second transparent substrate 32 are disposed, in turn, on the patterned spacer 20. Below the spacer 20, the first transparent substrate 12 is disposed.
The patterned spacer 20 formed by lithography can be disposed at non-display zones on demand and can be precisely controlled for its height. Therefore, such patterned spacers can impart high reliability in maintaining a distance between both substrates. Additionally, such patterned spacers have advantages in that it is possible to increase the solidity of a product due to the fixed position of a spacer and to prevent a so-called ripple phenomenon upon touching a screen.
There is no particular limitation in positions of the patterned spacer 20 as long as the patterned spacer 20 is disposed in non-display zones. Thus, the patterned spacer 20 is frequently disposed on the thin-film transistor T. Additionally, in the case of a high-aperture liquid crystal display device wherein edges of the first electrode 18 are overlapped with adjacent gate lines 14 and data lines 16, it is possible to mount the patterned spacer 20 on such overlapped first electrode.
When the above lithographic method is used, the above-mentioned problems occurring in the prior art can be solved by forming a spacer pattern in a position other than efficient pixel portions so as to prevent distortion of a desired liquid crystal array. Additionally, because spin coating conditions may be varied to form a coating layer with a variable thickness, it is possible to manufacture liquid crystal display panels with various modes having different cell intervals by using one kind of photosensitive resin composition for a patterned spacer.
Characteristics needed for photosensitive resin compositions for patterned spacers are as follows.
A patterned spacer should have such a high strength as to maintain the distance between a lower substrate and upper substrate.
Additionally, a patterned spacer is generally formed to have a coating layer with a thickness of 3 microns or more and a major portion of the coating layer should be developed. Therefore, a photosensitive resin composition for a patterned spacer should be dissolved into a developer solution in a short time and in a large amount. Further, when clear development is not made, various display defects such as spot formation due to residues after development and undesirable alignment of liquid crystals may occur. Therefore, the photosensitive resin composition should have excellent developability.
Meanwhile, when a glass substrate with large surface area is applied, it is difficult for the substrate to be subjected to full-surface exposure, and thus the substrate is subjected to exposure in multiple portions. Thus, when a photosensitive resin composition with low sensitivity is used, it is inevitable that the time needed for exposure becomes longer, resulting in a drop in the productivity. Therefore, a photosensitive resin composition for a patterned spacer should have high sensitivity.
Further, when carrying out a process of forming an alignment layer after a process of manufacturing a patterned spacer, it is necessary for the photosensitive resin composition to have excellent thermal stability for maintaining the original shape and thickness of a patterned spacer even under high temperature conditions of 200° C. or higher and to have high compress strength sufficient to resist against external pressure and excellent chemical resistance. In addition to the above, it is necessary for the photosensitive resin composition to have excellent stability with time so as to provide desirable characteristics stably without any changes even under long-term storage conditions.
Japanese Laid-Open Patent No. 2001˜151829 discloses a photosensitive resin composition for a patterned spacer. However, the photosensitive resin composition using a thermosetting binder has poor stability with time and low photosensitivity. Therefore, the composition is problematic in that it cannot realize a stable pattern under an exposure dose of 150 mJ/cm2 or less.
Therefore, it is desirable to provide a photosensitive resin composition having excellent strength, sensitivity, developability, thermal stability, chemical resistance and stability with time.